The present invention is based upon generation of cold plasma by electron cyclotron resonance heating in order to develop various desired characteristics within the plasma. Thus, the present invention and related prior art are based upon criteria where conditions governing steady-state values of plasma density and composition as well as the electron and ion temperatures are determined by microwave power and neutral gas pressure through the conditions for particle and power balances for each of three coupled species; namely electrons, ions and gas atoms.
Even in idealized point-model approximations of full transport models, inter-related values of the basic plasma parameters present for all such systems demonstrate key dependencies which indicate some of the fundamental obstacles which make it difficult in practice to achieve large volumes of quiescent, homogeneous, low-temperature plasma containing the desired concentration of reacting species which are obviously desirable or essential in a large variety of applications including negative ion sources for accelerators and more specifically, in plasma assisted semiconductor processing applications referred to in greater detail below.
Basic problems in forming suitable plasmas are also noted in the prior art references discussed below. In any event, the following discussion of the prior art and the following description of the present invention, partially based upon a comparison with the prior art, are intended to further demonstrate novelty in the method and apparatus of the present invention for overcoming problems or obstacles such as those generally referred to above and discussed in greater detail below.
In the semiconductor processing application referred to above, plasma sources employing electron cyclotron resonance (ECR) heating comprise an emerging or developing technology, for example in the deposition and etching of VLSI films. Such applications are typical of other processing technologies requiring the capability of achieving submicron feature dimensions, substantial processing rates and the capability of uniformly processing large specimens such as wafers. The general characteristics of these technologies are believed to be well understood by those skilled in the art and are accordingly not discussed in substantial detail herein.
As noted above, although the ECR plasma source of the present invention is contemplated for use in vapor deposition or etching applications, the general parameters of those applications are not of themselves a portion of the invention. Certain considerations in such applications are briefly summarized below in order to facilitate a more complete understanding of the invention.
In any event, ECR plasma sources such as those provided by the present invention and the prior art discussed below employ magnetic fields and microwave power to create chemically active plasmas, preferably at very low gas pressures. Low pressure operation is desirable in order to permit the formation of highly directional or anisotropic streams of low-temperature ions, radicals and other species of reactants which are uniform over substantial transverse dimensions larger than the sample being processed.
In an ECR plasma source designed for plasma-enhanced chemical vapor deposition or reactive ion etching of VLSI films, for example, a reactant gas is introduced into an evacuated chamber which is immersed in a steady magnetic field and exposed to electromagnetic radiation. The frequency of the radiation, f.sub..mu., is selected to resonate with the electron gyrofrequency in a region of the steady magnetic field called the resonant interaction region. The resonance condition relates the strength of the steady magnetic field in this region, B.sub.res, through the condition that f.sub..mu. =eB.sub.res /2.pi.m, where e and m are the magnitudes of the electric charge and mass of the electron, respectively.
Electrons in the resonant interaction region gain kinetic energy from the electromagnetic radiation, and if the radiation power and the gas pressure are suitably adjusted, the heated electrons may ionize the reactant gas molecules to create a plasma. The plasma ions and electrons flow out of the resonant interaction region and impinge on the VLSI film where the ions can be used for deposition of new materials or etching of existing films. If the plasma density is sufficiently high, the deposition or etch rates can be rapid, and if the ion and electron energies are sufficiently low, damage to the sample being processed can be prevented. For etching submicron-scale features, it is necessary for the ion trajectories to be highly directional. This is made possible by operating at sufficiently low gas pressures to ensure that the ion mean-free-path for scattering is longer than the distance to the specimens.
Additionally, if the temperature of the plasma ions is sufficiently low, and if there are no groups of energetic ions, the substrate can be biased electrically, relative to the plasma interior, to potentials of sufficient magnitude to insure highly anisotropic processing without exceeding a threshold ion energy above which excessive damage may occur.
In order to process specimens of commercial interest, it is further necessary that the stream of plasma from the ECR source be uniform over transverse dimensions larger than 15-20 cm. The present invention, as described below, addresses the need for large, uniform streams of low-temperature plasmas with high densities of ions and electrons in low-pressure neutral gas mixtures.
In the prior art, one class of ECR plasma sources is generally referred to as the "Sumitomo source". The Sumitomo source is illustrated for example by the plasma deposition apparatus of Matsuo, et al. U.S. Pat. No. 4,401,054 issued Aug. 30, 1983, and further discussed in S. Matsuo, M. Kiuchi and T. Ono, in Proceedings of the Tenth Symposium on IISIAT 1986, Tokyo, p. 471 and T. Ono, M. Oda, C. Takahashi and S. Matsuo, J. Vac. Sci. Technol. B4, 696 (1986).
In the apparatus of that patent, plasma flows toward the substrate or specimen along magnetic lines of force. As described below, it is very difficult to achieve a desired degree of spatial uniformity in the cold-plasma density. Furthermore, since electrons heated at the resonant interaction region where the magnetic intensity satisfies the aforementioned resonance condition) flow directly toward the specimen along these magnetic lines of force, it is necessary to limit applied microwave power in order to avoid the creation of unstable bursts of energetic electrons and associated groups of energetic ions which might damage the specimen. The physical processes responsible for generation of these unstable bursts of electrons and resultant energetic ions are discussed in the papers by Quon and Dandl; Guest, Fetzer and Dandl; and Dandl and Guest, which are incorporated in the experimental section below.
Related prior art references include U.S. Pat. No. 4,492,620 issued Jan. 8, 1985 to Matsuo, et al. and entitled "Plasma Deposition Method and Apparatus" and U.S. Pat. No. 4,564,997 issued Jan. 21, 1986 to Matsuo, et al. and entitled "Semiconductor Device and Manufacturing Process Thereof".
The above three patents issued under assignment to Nippon Telegraph & Telephone Public Corporation. Still other related references issued common assignment include U.S. Pat. No. 4,450,031 issued May 22, 1984 to Ono, et al. and entitled "Ion Shower Apparatus"; U.S. Pat. No. 4,503,807 issued Mar. 12, 1985 to Nakayama, et al. and entitled "Chemical Vapor Deposition Apparatus"; and U.S. Pat. No. 4,566,940 issued Jan. 28, 1986 to Itsumi, et al. and entitled "Manufacturing Process for Semiconductor Integrated Circuits".
Another design of a plasma source (of a very different fundamental nature) originated in France and was disclosed in U.S. Pat. No. 4,534,842 issued Aug. 13, 1985 to Arnal, et al. and entitled "Process and Device for Producing a Homogeneous Large-Volume Plasma of High Density and of Low Electronic Temperature". In the process and device of that patent, plasma was accumulated in a large volume free of magnetic fields to enhance spatial uniformity; however, the ECR heating region was localized to a small volume limiting the efficiency of plasma generation and thus the ratio of ion density to neutral gas density. Furthermore, energetic charged particles were not prevented from striking the chamber wall and thereby producing excessive densities of impurities.
Related references disclosing various concepts for ion sources include U.S. Pat. No. 3,571,734 issued Mar. 23, 1971; U.S. Pat. No. 3,774,001 issued Nov. 20, 1973; U.S. Pat. No. 3,790,787 issued Feb. 5, 1974; U.S. Pat. No. 4,417,178 issued Nov. 22, 1983; and U.S. Pat. No. 4,638,216 issued Jan. 20, 1987.
Still another set of references disclosed various designs for ion sources and included U.S. Pat. No. 3,500,077 issued Mar. 10, 1970; U.S. Pat. No. 3,582,849 issued Jun. 1, 1971; U.S. Pat. No. 3,660,715 issued May 2, 1972; U.S. Pat. No. 3,663,360 issued May 16, 1972; and U.S. Pat. No. 3,742,219 issued Jun. 26, 1973.
All of the above references as well as references cited therein are incorporated herein in order to assure a more complete understanding of the background for ECR plasma sources and applications therefor.
Accordingly, as was also noted above, there has been found to remain a need for ECR plasma sources useful in a variety of applications and capable of producing large, uniform streams of quiescent, low-temperature plasmas with high densities of ions and electrons, especially in low-pressure neutral-gas mixtures in order to assure unidirectionality or anisotropy of the plasma flow or flux, as well as the desired composition of reactant species.
Furthermore, different plasma-enhanced processing applications require that the plasma be enclosed in chambers whose walls are made of particular materials, such as fused quartz, stainless steel, nickel, aluminum, etc., depending on the specific requirements of the process for which the plasma is to be employed.